This invention relates generally to digital control fluid flow systems and specifically to fluid flow or fluid metering systems utilizing a pump as the fluid flow control mechanism.
Fluid metering systems have very broad applications from analytical chemistry, to microbiolgical firmentation, to medical anesthesia control. In general, most manufacturing processes involve some type of fluid metering.
Fluid metering designs in the past have often utilized pressure differentials through an orifice, or valve gating, or various types of pumps used in combination with orifices or valves to effect a controlled or determinable fluid volume controlled or determinable fluid volume flow rate, or to measure fluid volume flow rate in conjunction with a control valve.
More recently, the use of a pump whose motor speed is controllable has been developed as a positively controlled fluid metering technique. With this technique the direct control of pump operation results in the direct control of fluid. Dependence upon pressurized supplies or orifice sizing is eliminated. Thus, metering in the old sense of the term, i.e., measuring, as a feedback to input, has been minimized as a primary factor in metering control with these newer pumping systems.
These metering systems utilizing a pump for fluid flow control, however, monitor pump motor speed as an indication of the fluid metering rate. Inherent in these systems are inaccuracies and/or non-linearities due to check valve operation or pump fluid slippage or limited pump motor response to changes in electrical control signals which necessitates the monitoring actual pump motor speed.
Attempts have been made with the use of gear pumps using controlled DC motor drives with tachometer or servotype feedback control circuits. Such systems function acceptably at high motor speeds but at low speeds suffer from slippage around the gear teeth of the pump. More recently, systems have been developed using tubing pumps driven by DC motors and DC stepping motors. These systems suffer from tubing wear and non-linearities over moderate ranges of motor speed.
Presently, there is a very limited range of control offered by pump mechanized fluid metering systems. Often the maximum flow rate controllable is no more than ten times the minimum. This limits the capability of the system in proportioning. It also limits the ratio value for plural fluid flow to a factor of no more than 10 to 1.
An object of this invention is to provide a fluid metering system including a speed controlled pump as the metering means which has maximum to minimum range of 100,000 to 1, and which has a metering accuracy of within 0.1%.
A further object of this invention is to provide this system with a ratio or proportioning control circuit connected to operate multiple pumps for providing a desired fluid proportioning to an accuracy of within 0.1% and over a volumetric range of 100,000 to 1.
An even further object of this invention is to provide for a simplified input which determines volume and/or proportioning ratio.